The present disclosure relates to the technical field of modular multilevel converters (MMC) that convert alternating current (AC) into direct current (DC), and vice versa.
The disclosure relates more precisely to high voltage direct current (HVDC) transport networks that use DC for transmitting electricity and in which stations incorporate modular multilevel converters.
In FIG. 1, there can be seen a diagram showing a set 6 of submodules of a multilevel modular converter 10 of the prior art. For three-phase input/output (having three phases φa, φb, and φc), the converter 10 has three conversion legs, which are referenced by the indices a, b, and c given to the various components of FIG. 1. Each conversion leg comprises an upper arm and a lower arm (specified by the indices “u” for upper and “l” for lower), each of which connects one of the terminals DC+ or DC− of the DC power supply network to a terminal of the AC power supply network. In particular, each of the legs is connected to one of the three phase lines φa, φb, and φc, of the AC power supply network. FIG. 1 shows a set 6 of submodules, in which each arm passes a current ixi (with x specifying whether the arm is upper or lower, and with the index i specifying the leg). Also, in addition, each arm comprises a plurality of submodules SMxij that may be controlled with a desired sequence (where x specifies whether the arm is upper or lower, i specifies the phase line with which the arm is associated, and j is the number of the submodule from among the submodules in series in the arm). In this example, only three submodules are shown per arm. In practice, each lower or upper arm may have a number N of submodules that may lie in the range a few tens to a few hundreds. Each submodule SMxij includes an energy storage system such as at least one capacitor with a control member for selectively connecting the capacitor in series between the terminals of the submodule or for bypassing it. The submodules are controlled with a sequence that is selected so as to cause the number of energy storage elements that are connected in series in an arm of the converter 10 to vary progressively in order to deliver a plurality of voltage levels. Also, in FIG. 1, Vdc designates the voltage across the points where the converter is connected to the DC power supply network, with these points being known to the person skilled in the art as the “point of common coupling” (PCC). idc designates the current of the DC power supply network, while currents iga, igb, and igc are carried by the three phase lines φa, φb, and φc. Furthermore, each arm possesses an inductance Larm, and each phase line has an inductance Lf and a resistance Rf.
FIG. 2 shows a prior art submodule SMxij forming part of the FIG. 1 converter 10. In this submodule, each control member comprises a first electronic switch element T1 such as an insulated gate bipolar transistor (IGBT) connected in series with an electrical energy storage element, specifically a capacitor CSM. This first switch element T1 and this capacitor CSM are connected in parallel with a second electronic switch element T2, likewise an IGBT. This second electronic switch element of T2 is coupled between the inlet and outlet terminals of the submodule SMxij. Both of the first and second switch elements T1 and T2 are associated with respective antiparallel diodes, shown in FIG. 2.
In operation, the submodule may be controlled to occupy two control states.
In a first state, referred to as the “on” state, the first switch element T1 and the second switch element T2 are configured to connect the energy storage element CSM in series with the other submodules. In a second state, referred to as the “off” state, the first switch element T1 and the second switch element T2 are configured to short-circuit the energy storage element CSM.
It is known that each arm having a voltage vm across its terminals, can be modeled by a modeled voltage source having a voltage vm across its terminals, with a duty factor that depends on the number of submodules that are on, and by a modeled capacitor Ctot connected to the voltage source. This model is shown diagrammatically in FIG. 3, where there can be seen an arm passing a current i together with the model that is obtained. The reciprocal of the equivalent capacitance of the modeled capacitor Ctot is equal to the sum of the reciprocals of the capacitances of the modules that are on, such that:
      1          C      tot        =            1              C        1              +          1              C        2              +    ⋯    +          1              C        N            where C1, C2, . . . , Cj, . . . , CN is the capacitance of the jth capacitor.
Thus, the voltage vcΣ across the terminals of the modeled capacitor Ctot is equal to the sum of the voltages vcj across the terminals of the capacitors of the submodules in the arm (with j lying in the range 1 to N and giving the number of the capacitor and thus of the submodule). Also, each capacitor Ctot passes a current im. In the present application, by misuse of language, Ctot designates both the modeled capacitor and also its capacitance. By controlling the on/off sequence of the submodules, so as to cause the number of energy storage elements that are connected in series to vary progressively, it is possible to decrease or to increase the energy of the modeled capacitor Ctot and thus the voltage across the terminals of each modeled voltage source.
In the prior art, there is thus to be found a configuration equivalent to the set 6 of submodules of the MMC 10 as shown in FIG. 4. In this figure, the converter is a converter analogous to the converter described with reference to FIG. 1, and in which each arm has been replaced by its model. Also, each phase line of the AC power supply network is associated with a current igi and a voltage vgi (where the index i specifies the number of the leg).
In this example, each of the modeled voltage sources has a voltage vmxi across its terminals, and each of the modeled capacitors Ctot passes a current imxi and has across its terminals a voltage vcΣxi (where x specifies whether the arm is upper or lower and where i specifies the number of the leg). It can also be observed that it is possible to subdivide the MMC into a notional AC portion and a notional DC portion (at the inlet or the outlet, depending on whether the converter is configured to convert AC energy into a DC energy, or vice versa), in which the variation in the total energy stored in the capacitors of the submodules is equal to the difference between the power entering the converter and the power leaving it.
In this type of MMC, it is known that the internal energy stored in the capacitors of the submodules is decoupled from the voltage of the DC power supply network. Thus, the internal energy stored in the capacitors of MMCs can be regulated independently. This makes it possible in particular for MMCs to contribute to stabilizing associated DC and AC power supply networks by delivering or extracting energy to or from said power supply networks.
It can thus be understood that the exchanges of power between the DC and/or AC power supply networks and the MMC lead to an increase or to a decrease in the internal energy stored in the capacitors of the converter.
The internal energy of the converter has an impact on the stability of the DC and AC power supply networks. Also, it is known that the total voltage of the capacitors of the converter is caused to oscillate as a result of exchanges of power between the DC and AC power supply networks. These oscillations have the consequence of threatening proper operation of the converter by not complying with its operating constraints. Prior art solutions do not take these oscillations into account, thereby running the risk of damaging the converter. Those solutions therefore do not make it possible to take full advantage of the capabilities of MMCs in terms of controlling the internal energy of the converter.